Radiation detector capable of noise handling

ABSTRACT

Disclosed herein is a radiation detector, comprising: an avalanche photodiode (APD) with a first side coupled to an electrode and configured to work in a linear mode; a capacitor module electrically connected to the electrode and comprising a capacitor, wherein the capacitor module is configured to collect charge carriers from the electrode onto the capacitor; a current sourcing module in parallel to the capacitor, the current sourcing module configured to compensate for a leakage current in the APD and comprising a current source and a modulator; wherein the current source is configured to output a first electrical current and a second electrical current; wherein the modulator is configured to control a ratio of a duration at which the current source outputs the first electrical current to a duration at which the current source outputs the second electrical current.

TECHNICAL FIELD

The disclosure herein relates to radiation detectors, particularlyrelates to radiation detectors based on avalanche diodes capable ofnoise handling.

BACKGROUND

A radiation detector is a device that measures a property of aradiation. Examples of the property may include a spatial distributionof the intensity, phase, and polarization of the radiation. Theradiation may be one that has interacted with a subject. For example,the radiation measured by the radiation detector may be a radiation thathas penetrated or reflected from the subject. The radiation may be anelectromagnetic radiation such as infrared light, visible light,ultraviolet light, X-ray or γ-ray. The radiation may be of other typessuch as α-rays and β-rays.

One type of radiation detectors is based on interaction between theradiation and a semiconductor. For example, a radiation detector of thistype may have a semiconductor layer that absorbs the radiation andgenerate charge carriers (e.g., electrons and holes) and circuitry fordetecting the charge carriers.

Radiation detectors may be negatively impacted by “dark” noise (e.g.,leakage current). Dark noise in a radiation detector includes physicaleffects present even if no radiation the radiation detector isconfigured to detect is incident on the radiation detector. Isolating orreducing the impact of the dark noise to the overall signals detected bythe radiation detector is helpful to make the radiation detector moreuseful.

SUMMARY

Disclosed herein is a radiation detector, comprising: an avalanchephotodiode (APD) with a first side coupled to an electrode andconfigured to work in a linear mode; a capacitor module electricallyconnected to the electrode and comprising a capacitor, wherein thecapacitor module is configured to collect charge carriers from theelectrode onto the capacitor; a current sourcing module in parallel tothe capacitor, the current sourcing module configured to compensate fora leakage current of in the APD and comprising a current source and amodulator; wherein the current source is configured to output a firstelectrical current and a second electrical current; wherein themodulator is configured to control a ratio of a duration at which thecurrent source outputs the first electrical current to a duration atwhich the current source outputs the second electrical current.

According to an embodiment, the current sourcing module is adjustable.

According to an embodiment, the current sourcing module is configured todivert the leakage current of the APD through the current sourcingmodule.

According to an embodiment, the first electrical current and the secondelectrical current are different in their magnitude, direction, or both.

According to an embodiment, least one of the first electrical currentand the second electrical current is at least an order of magnitudelarger than the leakage current of the APD.

According to an embodiment, the electrical current of the dark noise isfrom 1 pA to 1000 pA.

According to an embodiment, the modulator comprises a processor or amemory.

According to an embodiment, the modulator comprises a switch.

According to an embodiment, the radiation comprises soft X-ray,ultraviolet (UV) light or extreme ultraviolet (EUV) light.

According to an embodiment, the current source comprises a currentmirror.

According to an embodiment, the modulator is located on an input stageof the current mirror.

According to an embodiment, the modulator comprises a current sourceconfigured to output electrical current at alternating magnitudes.

According to an embodiment, the modulator comprises a current sourceconfigured to output two magnitudes of electrical current withadjustable ratio of durations.

According to an embodiment, the modulator is located on an output stageof the current mirror.

According to an embodiment, the modulator comprises a switch configuredto controllably connect the current sourcing module to and tocontrollably disconnect it from the capacitor.

According to an embodiment, the radiation detector further comprises: afirst voltage comparator configured to compare a voltage of theelectrode to a first threshold; a second voltage comparator configuredto compare the voltage to a second threshold; a counter configured toregister a number of photons absorbed by the absorption layer; acontroller; wherein the controller is configured to start a time delayfrom a time at which the first voltage comparator determines that anabsolute value of the voltage equals or exceeds an absolute value of thefirst threshold; wherein the controller is configured to activate thesecond voltage comparator during (including the beginning and theexpiration) the time delay; wherein the controller is configured tocause the number registered by the counter to increase by one, if thesecond voltage comparator determines that an absolute value of thevoltage equals or exceeds an absolute value of the second threshold.

According to an embodiment, the controller is configured to activate thesecond voltage comparator at a beginning or expiration of the timedelay.

According to an embodiment, the apparatus further comprises a voltmeterand the controller is configured to cause the voltmeter to measure thevoltage upon expiration of the time delay.

According to an embodiment, the controller is configured to determine anX-ray photon energy based on a value of the voltage measured uponexpiration of the time delay.

According to an embodiment, the controller is configured to connect theelectrode to an electrical ground.

According to an embodiment, a rate of change of the voltage issubstantially zero at expiration of the time delay.

According to an embodiment, a rate of change of the voltage issubstantially non-zero at expiration of the time delay.

According to an embodiment, the apparatus comprises an array of APDs.

Disclosed herein is a system comprising the apparatus described aboveand an X-ray source, wherein the system is configured to perform X-rayradiography on human chest or abdomen.

According to an embodiment, the system comprises the apparatus describedabove and an X-ray source, wherein the system is configured to performX-ray radiography on human mouth.

Disclosed herein is a cargo scanning or non-intrusive inspection (NII)system, comprising the apparatus described above and an X-ray source,wherein the cargo scanning or non-intrusive inspection (NII) system isconfigured to form an image using backscattered X-ray.

Disclosed herein is a cargo scanning or non-intrusive inspection (NII)system, comprising the apparatus described above and an X-ray source,wherein the cargo scanning or non-intrusive inspection (NII) system isconfigured to form an image using X-ray transmitted through an objectinspected.

Disclosed herein is a full-body scanner system comprising the apparatusdescribed above and an X-ray source.

Disclosed herein is an X-ray computed tomography (X-ray CT) systemcomprising the apparatus described above and an X-ray source.

Disclosed herein is an electron microscope comprising the apparatusdescribed above, an electron source and an electronic optical system.

Disclosed herein is a system comprising the apparatus described above,wherein the system is an X-ray telescope, or an X-ray microscopy, orwherein the system is configured to perform mammography, industrialdefect detection, microradiography, casting inspection, weld inspection,or digital subtraction angiography.

Disclosed herein is a method comprising: determining a contribution of aleakage current in signals of an avalanche photodiode (APD) working in alinear mode; determining a ratio of a duration of a first compensatorysignal to a duration of a second compensatory signal based on thecontribution of the leakage current, the first compensatory signal andthe second compensatory signal; and compensating the signals of the APDfor the leakage current with the first compensatory signal and thesecond compensatory signal with their respective durations with theratio.

According to an embodiment, the contribution is determined by measuringthe signals while the APD receives no radiation.

According to an embodiment, the first compensatory signal and the secondcompensatory signal are electrical currents.

Disclosed herein is a method comprising: measuring signals of anavalanche photodiode (APD) working in a linear mode when the APDreceives no radiation and a compensation for a leakage current of theAPD is present; if the signals have exceeded a first level, commencing atime delay; measuring the signals of the APD at an end of the timedelay; and if the signals at the end of the time delay exceed a secondlevel, increasing the compensation for the leakage current.

According to an embodiment, the compensation is increased to a magnitudeamong a group of discrete values.

According to an embodiment, the method further comprises: if the signalsat the end of the time delay exceed a second level, resetting thesignals.

Disclosed herein is a method comprising: measuring signals of anavalanche photodiode (APD) working in a linear mode when the APDreceives no radiation and a compensation for a leakage current of theAPD is present; if the signals have exceeded a first level, commencing atime delay; measuring the signals of the APD at an end of the timedelay; determining a difference between the signals at the end of thetime delay and the signals at the beginning of the time delay; anddetermining a magnitude of the compensation based on the difference.

BRIEF DESCRIPTION OF FIGURES

FIG. 1A schematically shows the current-voltage characteristics of anAPD in the linear mode, and in the Geiger mode.

FIG. 1B schematically shows the electric current in an APD as a functionof the intensity of light incident on the APD when the APD is in thelinear mode, and a function of the intensity of light incident on theAPD when the APD is in the Geiger mode.

FIG. 2A, FIG. 2B and FIG. 2C schematically show the operation of an APD,according to an embodiment.

FIG. 3A schematically shows a cross section of a radiation detectorbased on an array of APDs.

FIG. 3B shows a variant of the radiation detector of FIG. 3A.

FIG. 3C shows a variant of the radiation detector of FIG. 3A.

FIG. 3D shows a variant of the radiation detector of FIG. 3A.

FIG. 4A and FIG. 4B schematically show a cross-sectional view of aradiation detector comprising a plurality of APDs.

FIG. 5A and FIG. 5B schematically show a cross-sectional view of aradiation detector comprising a plurality of APDs.

FIG. 6A and FIG. 6B each show a component diagram of an electronicsystem of a radiation detector, according to an embodiment.

FIG. 7A and FIG. 7B respectively show a circuit configured to compensatefor the dark noise in the form of an electrical current.

FIG. 8 schematically shows the current sourcing module in the electronicsystem of the radiation detector, according to an embodiment.

FIG. 9 and FIG. 10 show two examples of the current sourcing module,where the current source of the current sourcing module includes acurrent mirror.

FIG. 11 schematically shows the electrical current the current sourcingmodule sources, the voltage across the capacitor of the capacitor moduleattributable to the dark noise and the electrical current the currentsourcing module provides, the voltage across the capacitor of thecapacitor module attributable to only the dark noise, as functions oftime.

FIG. 12 schematically shows a voltage across the capacitor as a functionof time, where the capacitor module includes the current sourcingmodule.

FIG. 13 schematically shows a flow chart for a method of compensatingfor dark noise in a radiation detector.

FIG. 14A schematically shows a flow chart for a method of compensatingfor dark noise in a radiation detector.

FIG. 14B schematically shows a flow chart for a method of compensatingfor dark noise in a radiation detector.

FIG. 15 -FIG. 21 each schematically show a system comprising theradiation detector described herein.

DETAILED DESCRIPTION

An avalanche photodiode (APD) is a photodetector based on semiconductormaterial. For example, an APD may be in a form of a p-n junction under areverse bias (i.e., the p-type region of the p-n junction is biased at alower electric potential than the n-type region). The p-n junction mayhave a breakdown voltage. The breakdown voltage is a reverse bias, abovewhich exponential increase in the electric current in the p-n junctionmay occur.

An APD may operate at one of two modes. In one mode, the reverse bias ofthe p-n junction may be above the breakdown voltage. Here the word“above” means that absolute value of the reverse bias is greater thanthe absolute value of the breakdown voltage. This mode may be referredto as Geiger-mode and an APD working in this mode may be referred to asa single-photon avalanche diode (SPAD) (also known as a Geiger-mode APDor G-APD). In another mode, the reverse bias of the p-n junction may bebelow the breakdown voltage and this mode may be referred to as linearmode.

When a photon (e.g., visible light, ultraviolet or extreme ultraviolet(EUV) light) incidents on an APD, it may generate charge carriers(electrons and holes). Some of the charge carriers may be accelerated byan electric field in the APD and may trigger a current by impactionization (e.g., an avalanche current in the case of a SPAD). Impactionization is a process in a material by which one energetic chargecarrier can lose energy by the creation of other charge carriers. Forexample, in semiconductors, an electron (or hole) with enough kineticenergy can knock a bound electron out of its bound state (in the valenceband) and promote it to a state in the conduction band, creating anelectron-hole pair.

FIG. 1A schematically shows the current-voltage characteristics 100 ofan APD in the linear mode, and in the Geiger mode (i.e., when the APD isa SPAD). The APD may have a bifurcation of the current-voltagecharacteristics 100 above the breakdown voltage V_(BD) (i.e., a SPAD).When the reverse biased is above V_(BD), both electrons and holes maycause significant ionization, and the avalanche is self-sustaining. Whenthe avalanche is triggered (e.g., by an incident photon) at a reversebiased is above V_(BD), the avalanche current is sustained (“on-branch”110); when the avalanche is not triggered at a reverse biased is aboveV_(BD), very little electric current flows through (“off-branch” 120).At a reverse bias above V_(BD), when an incident photon triggersavalanche in the APD, the current-voltage characteristics 100 of the APDtransitions (as indicated by the arrow 130) from the off-branch 120 tothe on-branch 110. This transition manifests as a sharp increase ofelectric current flowing through the APD, from essentially zero to afinite value of I_(L). This transition is similar to the mechanismbehind the Geiger counter. Therefore, at a reverse bias above V_(BD), anAPD is operating in the “Geiger mode.” An APD working at a reverse biasbelow the breakdown voltage is operating in the linear mode because theelectric current in the APD is proportional to the intensity of thelight incident on the APD.

FIG. 1B schematically shows the electric current in an APD as a function112 of the intensity of light incident on the APD when the APD is in thelinear mode, and a function 111 of the intensity of light incident onthe APD when the APD is in the Geiger mode (i.e., when the APD is aSPAD). In the Geiger mode, the current shows a very sharp increase withthe intensity of the light and then saturation. In the linear mode, thecurrent is essentially proportional to the intensity of the light.

FIG. 2A, FIG. 2B and FIG. 2C schematically show the operation of an APD,according to an embodiment. FIG. 2A shows that when a photon (e.g., anX-ray photon) is absorbed by an absorption region 202, multiple (100 to10000 for an X-ray photon) electron-hole pairs maybe generated. Theabsorption region 202 has a sufficient thickness and thus a sufficientabsorptance (e.g., >80% or >90%) for the incident photon. For soft X-rayphotons, the absorption region 202 may be a silicon layer with athickness of 10 microns or above. The electric field in the absorptionregion 202 is not high enough to cause avalanche effect in theabsorption region 202. FIG. 2B shows that the electrons and hole driftin opposite directions in the absorption region 202. FIG. 2C shows thatavalanche effect occurs in an amplification region 204 when theelectrons (or the holes) enter that amplification region 204, therebygenerating more electrons and holes. The electric field in theamplification region 204 is high enough to cause an avalanche of chargecarriers entering the amplification region 204 but not too high to makethe avalanche effect self-sustaining. A self-sustaining avalanche is anavalanche that persists after the external triggers disappear, such asphotons incident on the APD or charge carriers drifted into the APD. Theelectric field in the amplification region 204 may be a result of adoping profile in the amplification region 204. For example, theamplification region 204 may include a p-n junction or a heterojunctionthat has an electric field in its depletion zone. The threshold electricfield for the avalanche effect (i.e., the electric field above which theavalanche effect occurs and below which the avalanche effect does notoccur) is a property of the material of the amplification region 204.The amplification region 204 may be on one or two opposite sides of theabsorption region 202.

FIG. 3A schematically shows a cross section of a radiation detector 300based on an array of APDs 350. Each of the APDs 350 may have anabsorption region 310 and an amplification region 320 as the exampleshown in FIG. 2A, FIG. 2B and FIG. 2C. At least some, or all, of theAPDs 350 in the radiation detector 300 may have their absorption regions310 joined together. Namely, the radiation detector 300 may have joinedabsorption regions 310 in a form of an absorption layer 311 that isshared among at least some or all of the APDs 350. The amplificationregions 320 of the APDs 350 are discrete regions. Namely theamplification regions 320 of the APDs 350 are not joined together. In anembodiment, the absorption layer 311 may be in form of a semiconductorwafer such as a silicon wafer. The absorption regions 310 may be anintrinsic semiconductor or very lightly doped semiconductor (e.g., <10¹²dopants/cm³, <10¹¹ dopants/cm³, <10¹⁰ dopants/cm³, <10⁹ dopants/cm³),with a sufficient thickness and thus a sufficient absorptance(e.g., >80% or >90%) for incident photons of interest (e.g., X-rayphotons). The amplification regions 320 may have a junction 315 formedby at least two layers 312 and 313. The junction 315 may be aheterojunction of a p-n junction. In an embodiment, the layer 312 is ap-type semiconductor (e.g., silicon) and the layer 313 is a heavilydoped n-type layer (e.g., silicon). The phrase “heavily doped” is not aterm of degree. A heavily doped semiconductor has its electricalconductivity comparable to metals and exhibits essentially linearpositive thermal coefficient. In a heavily doped semiconductor, thedopant energy levels are merged into an energy band. A heavily dopedsemiconductor is also called degenerate semiconductor. The layer 312 mayhave a doping level of 10¹³ to 10¹⁷ dopants/cm³. The layer 313 may havea doping level of 10¹⁸ dopants/cm³ or above. The layers 312 and 313 maybe formed by epitaxy growth, dopant implantation or dopant diffusion.The band structures and doping levels of the layers 312 and 313 can beselected such that the depletion zone electric field of the junction 315is greater than the threshold electric field for the avalanche effectfor electrons (or for holes) in the materials of the layers 312 and 313,but is not too high to cause self-sustaining avalanche. Namely, thedepletion zone electric field of the junction 315 should cause avalanchewhen there are incident photons in the absorption region 310 but theavalanche should cease without further incident photons in theabsorption region 310.

The radiation detector 300 may further include electrodes 304respectively in electrical contact with the layer 313 of the APDs 350.The electrodes 304 are configured to collect electric current flowingthrough the APDs 350.

The radiation detector 300 may further include a passivation material303 configured to passivate surfaces of the absorption regions 310 andthe layer 313 of the APDs 350 to reduce recombination at these surfaces.

The radiation detector 300 may further include a heavily doped layer 302disposed on the absorption regions 310 opposite to the amplificationregion 320, and a common electrode 301 on the heavily doped layer 302.The common electrode 301 of at least some or all of the APDs 350 may bejoined together. The heavily doped layer 302 of at least some or all ofthe APDs 350 may be joined together.

When a photon (e.g., visible light, violet, ultraviolet or extremeultraviolet (EUV)) incidents on the radiation detector 300, it may beabsorbed by the absorption region 310 of one of the APDs 350, and chargecarriers may be generated in the absorption region 310 as a result. Onetype (electrons or holes) of the charge carriers drift toward theamplification region 320 of that one APD. When the charge carriers enterthe amplification region 320, the avalanche effect occurs and causesamplification of the charge carriers. The amplified charge carriers canbe collected through the electrode 304 of that one APD, as an electriccurrent. When that one APD is in the linear mode, the electric currentis proportional to the number of incident photons in the absorptionregion 310 per unit time (i.e., proportional to the light intensity atthat one APD). The electric currents at the APDs may be compiled torepresent a spatial intensity distribution of light, i.e., an image. Theamplified charge carriers may alternatively be collected through theelectrode 304 of that one APD, and the number of photons may bedetermined from the charge carriers (e.g., by using the temporalcharacteristics of the electric current).

The junctions 315 of the APDs 350 should be discrete, i.e., the junction315 of one of the APDs should not be joined with the junction 315 ofanother one of the APDs. Charge carriers amplified at one of thejunctions 315 of the APDs 350 should not be shared with another of thejunctions 315. The junction 315 of one of the APDs may be separated fromthe junction 315 of the neighboring APDs by the material of theabsorption region wrapping around the junction, by the material of thelayer 312 or 313 wrapping around the junction, by an insulator materialwrapping around the junction, or by a guard ring of a dopedsemiconductor. As shown in FIG. 3A, the layer 312 of each of the APDs350 may be discrete, i.e., not joined with the layer 312 of another oneof the APDs; the layer 313 of each of the APDs 350 may be discrete,i.e., not joined with the layer 313 of another one of the APDs. FIG. 3Bshows a variant of the radiation detector 300, where the layers 312 ofsome or all of the APDs are joined together. FIG. 3C shows a variant ofthe radiation detector 300, where the junction 315 is surrounded by aguard ring 316. The guard ring 316 may be an insulator material or adoped semiconductor. For example, when the layer 313 is heavily dopedn-type semiconductor, the guard ring 316 may be n-type semiconductor ofthe same material as the layer 313 but not heavily doped. The guard ring316 may be present in the radiation detector 300 shown in FIG. 3A orFIG. 3B. FIG. 3D shows a variant of the radiation detector 300, wherethe junction 315 has an intrinsic semiconductor layer 317 sandwichedbetween the layer 312 and 313. The intrinsic semiconductor layer 317 ineach of the APDs 350 may be discrete, i.e., not joined with otherintrinsic semiconductor layer 317 of another APD. The intrinsicsemiconductor layers 317 of some or all of the APDs 350 may be joinedtogether.

FIG. 4A and FIG. 4B schematically show a cross-sectional view of aradiation detector 500 comprising a plurality of APDs 511. The APDs 511may be fabricated in a substrate 510 (e.g., a semiconductor wafer). Oneor more vias 512 may be present in the substrate 510 and the vias 512electrically connect the APDs 511 to a surface of the substrate 510.Alternatively, the APDs 511 may be disposed on the surface of thesubstrate 510 such that electrical contacts on the APDs 511 are exposedto the surface. Electronic systems 521 that communicate and/or controlthe APDs 511 may be fabricated in another substrate 520. Electronicsystems 521 may include controllers, bias sources, switches, currentmeters, memories, amplifiers or other suitable components. Somecomponents of the electronic systems 521 may be fabricated in thesubstrate 510. Electronic systems 521 may be configured to use the APDs511 using the method illustrated in FIG. 3 . One or more vias 522 may bepresent and electrically connect the electronic systems 521 to a surfaceof the substrate 520. Alternatively, the electronic systems 521 may bedisposed at the surface of the substrate 520 such that electricalcontacts on the electronic systems 521 are exposed to the surface. Thesubstrate 520 may include transmission lines 530 configured to transmitdata, power and/or signals to and from the electronic systems 521, andthrough which to and from the APDs 511. The substrates 510 and 520 maybe bonded by a suitable substrate bonding technique, such as flip chipbonding or direct bonding.

As shown in FIG. 4A and FIG. 4B, flip chip bonding uses solder bumps 599deposited onto the surface of either one of the substrates 510 and 520.Either of the substrates 510 and 520 is flipped over and the APDs 511and the electronic systems 521 are aligned (e.g., through the vias 512,522 or both). The substrates 510 and 520 are brought into contact. Thesolder bumps 599 may be melted to electrically connect the APDs 511 andthe electronic systems 521. Any void space among the solder bumps 599may be filled with an insulating material.

Direct bonding is a wafer bonding process without any additionalintermediate layers (e.g., solder bumps). The bonding process is basedon chemical bonds between two surfaces. Direct bonding may be atelevated temperature but not necessarily so.

FIG. 5A and FIG. 5B schematically show a cross-sectional view of aradiation detector 600 comprising a plurality of APDs 611. The APDs 611may be fabricated in a substrate 610 (e.g., a semiconductor wafer). Oneor more vias 612 may be present in the substrate 610 and the vias 612electrically connect the APDs 611 to a surface of the substrate 610.Alternatively, the APDs 611 may be disposed on the surface of thesubstrate 610 such that electrical contacts on the APDs 611 are exposedto the surface. The substrate 610 may include transmission lines 630.Electronic systems 621 that communicate and/or control the APDs 611 maybe fabricated in another substrate 620. Electronic systems 621 mayinclude controllers, bias sources, switches, current meters, memories,amplifiers or other suitable components. Some components of theelectronic systems 621 may be fabricated in the substrate 610.Electronic systems 621 may be configured to use the APDs 611 using themethod illustrated in FIG. 3 . One or more vias 622 and 623 may bepresent and electrically connect the electronic systems 621 to a surfaceof the substrate 620. Alternatively, the electronic systems 621 may bedisposed at the surface of the substrate 620 such that electricalcontacts on the electronic systems 621 are exposed to the surface. Thesubstrates 610 and 620 may be bonded by a suitable substrate bondingtechnique, such as flip chip bonding or direct bonding.

As shown in FIG. 5A and FIG. 5B, flip chip bonding uses solder bumps 699and 698 deposited onto the surface of either one of the substrates 610and 620. Either of the substrates 610 and 620 is flipped over and theAPDs 611 and the electronic systems 621 are aligned (e.g., through thevias 612, 622 or both). The substrates 610 and 620 are brought intocontact. The solder bumps 699 may be melted to electrically connect theAPDs 611 and the electronic systems 621. The solder bumps 698 may bemelted to electrically connect the electronic systems 621 to thetransmission lines 630. The transmission lines 630 configured totransmit data, power and/or signals to and from the electronic systems621, and through which to and from the APDs 611. Any void space amongthe solder bumps 699 and 698 may be filled with an insulating material.

It should be noted that the APDs in a radiation detector according to anembodiment (e.g., the radiation detector 300, 500 and/or 600) may workin the linear mode. The signals generated by the radiation incident onthe radiation absorption layer (e.g., 210 or 311) may be in a form of anelectrical current. Likewise, the dark noise may also be in a form of anelectrical current (e.g., a DC current or leakage current flowing fromthe electric contacts 219B or electrodes 304). If the current may beascertained, the electrical current may be compensated for (e.g.,diverted from) the electronic system (e.g., 221, 521 or 621).

FIG. 6A and FIG. 6B each show a component diagram of the electronicsystem 700 according to an embodiment. The system 700 may be any one ofthe electronic systems described herein, for example, 221, 521 or 621.The system includes a capacitor module 709 electrically connected to anelectrode of a diode 704 or an electrical contact, wherein the capacitormodule is configured to collect charge carriers from the electrode. Thediode 704 may be an exemplary APD as described herein and the electricalcontact may be an electrical contact of an exemplary APD as describedherein. The capacitor module can include a capacitor and charge carriersfrom the electrode accumulate on the capacitor over a period of time(“integration period”). After the integration period has expired, thecapacitor voltage is sampled and then reset by a reset switch. Thecapacitor module can include a capacitor directly connected to theelectrode. The capacitor may be in the feedback path of an amplifier.The amplifier configured as such is called a capacitive transimpedanceamplifier (CTIA). CTIA has high dynamic range by keeping the amplifierfrom saturating and improves the signal-to-noise ratio by limiting thebandwidth in the signal path.

The dark noise in the form of an electrical current, if not compensatedfor, charges the capacitor in the capacitor module 709 along with thesignals generated by the radiation.

FIG. 7A and FIG. 7B respectively show a circuit configured to provide anelectrical current to compensate for the dark noise. Dark noise in thesemiconductor devices may include leakage current in the semiconductormaterials, for example, leakage current in an APD. A current sourcingmodule 788 is in parallel to the capacitor. The current sourcing module788 may be adjustable such that the electrical current it sourcescompensates for the electrical current of the dark noise. In the circuitshown in FIG. 7A and FIG. 7B, the electrical current of the dark noiseis diverted through the current sourcing module 788 so that theelectrical current of the dark noise does not charge the capacitor.

The electrical current of the dark noise may be a very small current,such as in the range of picoamps (e.g., 1-1000 pA). Compensating for asmall electrical current may be challenging. FIG. 8 schematically showsthe current sourcing module 788, according to an embodiment. The currentsourcing module 788 may include a current source 901 and a modulator902. The current source 901 is configured to output a first electricalcurrent and a second electrical current. The first electrical currentand the second electrical current are different in their magnitude,direction, or both. The modulator 902 controls the ratio of the durationat which the current source 901 outputs the first electrical current tothe duration at which the current source 901 outputs the secondelectrical current. The first electrical current and the secondelectrical current may not be as small as the electrical current of thedark noise but the temporal average of the electrical current thecurrent sourcing module 788 sources, as a result of the modulation bythe modulator 902, may be equal to the electrical current of the darknoise. For example, at least one of the first electrical current and thesecond electrical current is at least an order of magnitude larger thanthe electrical current of the dark noise. For example, if the firstelectrical current is 1 nA and the second electrical current is 0, andthe ratio is 1:999, the temporal average of the electrical current thecurrent sourcing module 788 sources is 1 pA. The modulator 902 may be assimple as a switch. The modulator 902 may have complex circuitry such asa processor or a memory.

FIG. 9 and FIG. 10 show two examples of the current sourcing module 788,where the current source 901 includes a current mirror. A current mirroris a circuit that receives an input electrical current and outputs anoutput electrical current proportional to the input electrical current.A current mirror can be viewed as a current-controlled current source(CCCS). A current mirror may include two cascaded current-to-voltage andvoltage-to-current converters placed at the same conditions and havingreverse characteristics. A current mirror may be implemented usingMOSFET transistors as shown here. A current mirror may be implementedusing bipolar junction transistors. The modulator 902 may be located onthe output stage of the current mirror, as shown in FIG. 9 . Forexample, the modulator 902 may include a switch that controllablyconnects the current sourcing module 788 to and disconnects it from thecapacitor in the capacitor module 709. The modulator 902 may be locatedon the input stage of the current mirror, as shown in FIG. 10 . Themodulator 902 may include a current source outputting electrical currentat alternating magnitudes. The modulator 902 may include a currentsource outputting two magnitudes of electrical current with adjustableratio of durations.

FIG. 11 schematically shows an electrical current 1201 of the currentsourcing module 788 sources, as a function of time. The dashed line 1202shows the temporal average of the electrical current 1201. FIG. 11 alsoschematically shows the voltage 1203 across the capacitor of thecapacitor module 709 attributable to the dark noise and the electricalcurrent the current sourcing module 788 provides, as a function of time.FIG. 11 also schematically shows the voltage 1204 across the capacitorof the capacitor module 709 attributable to only the dark noise, as afunction of time. It can be observed from FIG. 11 that the electricalcurrent the current sourcing module 788 provides, on temporal average,removes the effect of the dark noise on the voltage across thecapacitor.

FIG. 12 schematically shows a voltage across the capacitor as a functionof time, where the capacitor module 709 includes the current sourcingmodule 788. A fine saw tooth waveform superimposed on a smoothlychanging voltage can be seen in FIG. 12 . The saw tooth waveform isattributable to the dark noise and the electrical current the currentsourcing module 788 provides, as a function of time.

FIG. 13 schematically shows a flow chart for a method of compensatingfor dark noise in a radiation detector. In procedure 2010, acontribution 2020 of a dark noise in the signals of the radiationdetector is determined. For example, the contribution may be determinedby measuring the signals while the radiation detector receives noradiation. In procedure 2030, a ratio 2040 of a duration of a firstcompensatory signal 2050 to a duration of a second compensatory signal2060 is determined based on the contribution 2020 of the dark noise, thefirst compensatory signal 2050 and the second compensatory signal 2060.For example, the first compensatory signal 2050 and the secondcompensatory signal 2060 may be the first electrical current and thesecond electrical current output by the current source 901. In procedure2070, the signals of the radiation detector are compensated for the darknoise with the first compensatory signal 2050 and the secondcompensatory signal 2060 with their respective durations with the ratio2040.

FIG. 14A schematically shows a flow chart for a method of compensatingfor dark noise in a radiation detector. In procedure 2110, signals ofthe radiation detector are measured, when the radiation detectorreceives no radiation and a compensation for the dark noise of theradiation detector is present. In procedure 2120, if the signals havenot exceeded a first level, the flow goes back the procedure 2110; ifthe signals have exceeded the first level, a time delay is commenced inprocedure 2130. In procedure 2140, the signals of the radiation detectorat the end of the time delay are measured. In procedure 2150, if thesignals do not exceed a second level, the flow ends and the currentmagnitude of compensation is deemed sufficient to compensate for thecontribution of the dark noise; if the signals at the end of the timedelay exceed the second level, the compensation for the dark noise isincreased in procedure 2160, the signals are reset in procedure 2170,and the flow goes back the procedure 2110. Alternatively, in procedure2150, if the signals do not exceed a second level, the second level islowered in procedure 2180 and the flow goes back the procedure 2110; ifthe signals at the end of the time delay exceed the second level, thecompensation for the dark noise is increased in procedure 2160, thesignals are reset in procedure 2170, and the flow goes back theprocedure 2110. When the compensation for the dark noise is increased,it may be increased to a magnitude among a group of discrete values. Thecurrent magnitude of compensation may be stored in a memory in theradiation detector.

FIG. 14B schematically shows a flow chart for a method of compensatingfor dark noise in a radiation detector. In procedure 2210, signals ofthe radiation detector are measured, when the radiation detectorreceives no radiation and a compensation for the dark noise of theradiation detector is present. In procedure 2220, if the signals havenot exceeded a first level, the flow goes back the procedure 2210; ifthe signals have exceeded the first level, a time delay is commenced inprocedure 2230. In procedure 2240, if the signals of the radiationdetector at the end of the time delay are measured. In procedure 2250,the difference of the signals at the beginning of the time delay (whichmay simply be the first level) and the signals at the end of the timedelay is determined. In procedure 2260, the magnitude of thecompensation for the dark noise is determined based on the difference.

In addition the capacitor module 709, which includes the currentsourcing module 788, the electronic system 700 may further include afirst voltage comparator 701, a second voltage comparator 702, a counter720, a switch 705, a voltmeter 706 and a controller 710, as shown inFIG. 6A and FIG. 6B.

The first voltage comparator 701 is configured to compare the voltage ofan electrode of a diode 704 to a first threshold. The diode may be anAPD as described herein. Alternatively, the first voltage comparator 701is configured to compare the voltage of an electrical contact (e.g., adiscrete portion of electrical contact 219B) to a first threshold. Thefirst voltage comparator 701 may be configured to monitor the voltagedirectly, or calculate the voltage by integrating an electric currentflowing through the diode or electrical contact over a period of time.The first voltage comparator 701 may be controllably activated ordeactivated by the controller 710. The first voltage comparator 701 maybe a continuous comparator. Namely, the first voltage comparator 701 maybe configured to be activated continuously, and monitor the voltagecontinuously. The first voltage comparator 701 configured as acontinuous comparator reduces the chance that the system 700 missessignals generated by an incident photon. The first voltage comparator701 configured as a continuous comparator is especially suitable whenthe incident radiation intensity is relatively high. The first voltagecomparator 701 may be a clocked comparator, which has the benefit oflower power consumption. The first voltage comparator 701 configured asa clocked comparator may cause the system 700 to miss signals generatedby some incident photons. When the incident radiation intensity is low,the chance of missing an incident photon is low because the timeinterval between two successive photons is relatively long. Therefore,the first voltage comparator 701 configured as a clocked comparator isespecially suitable when the incident radiation intensity is relativelylow. The first threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50%of the maximum voltage one incident photon may generate in the APD. Themaximum voltage may depend on the energy of the incident photon (i.e.,the wavelength of the incident radiation), the material of the radiationabsorption layer, and other factors. For example, the first thresholdmay be 50 mV, 100 mV, 150 mV, or 200 mV.

The second voltage comparator 702 is configured to compare the voltageto a second threshold. The second voltage comparator 702 may beconfigured to monitor the voltage directly, or calculate the voltage byintegrating an electric current flowing through the diode or theelectrical contact over a period of time. The second voltage comparator702 may be a continuous comparator. The second voltage comparator 702may be controllably activate or deactivated by the controller 710. Whenthe second voltage comparator 702 is deactivated, the power consumptionof the second voltage comparator 702 may be less than 1%, less than 5%,less than 10% or less than 20% of the power consumption when the secondvoltage comparator 702 is activated. The absolute value of the secondthreshold is greater than the absolute value of the first threshold. Asused herein, the term “absolute value” or “modulus” |x| of a real numberx is the non-negative value of x without regard to its sign. Namely,

${❘x❘} = \left\{ {\begin{matrix}{x,{{{if}x} \geq 0}} \\{{- x},{{{if}x} \leq 0}}\end{matrix}.} \right.$

The second threshold may be 200%-300% of the first threshold. The secondthreshold may be at least 50% of the maximum voltage one incident X-rayphoton may generate in the diode or resistor. For example, the secondthreshold may be 100 mV, 150 mV, 200 mV, 250 mV or 300 mV. The secondvoltage comparator 702 and the first voltage comparator 701 may be thesame component. Namely, the system 700 may have one voltage comparatorthat can compare a voltage with two different thresholds at differenttimes.

The first voltage comparator 701 or the second voltage comparator 702may include one or more op-amps or any other suitable circuitry. Thefirst voltage comparator 701 or the second voltage comparator 702 mayhave a high speed to allow the system 700 to operate under a high fluxof incident X-ray. However, having a high speed is often at the cost ofpower consumption.

The counter 720 is configured to register a number of X-ray photonsreaching the diode or resistor. The counter 720 may be a softwarecomponent (e.g., a number stored in a computer memory) or a hardwarecomponent (e.g., a 4017 IC and a 7490 IC).

The controller 710 may be a hardware component such as a microcontrollerand a microprocessor. The controller 710 is configured to start a timedelay from a time at which the first voltage comparator 701 determinesthat the absolute value of the voltage equals or exceeds the absolutevalue of the first threshold (e.g., the absolute value of the voltageincreases from below the absolute value of the first threshold to avalue equal to or above the absolute value of the first threshold). Theabsolute value is used here because the voltage may be negative orpositive, depending on whether the voltage of the cathode or the anodeof the diode or which electrical contact is used. The controller 710 maybe configured to keep deactivated the second voltage comparator 702, thecounter 720 and any other circuits the operation of the first voltagecomparator 701 does not require, before the time at which the firstvoltage comparator 701 determines that the absolute value of the voltageequals or exceeds the absolute value of the first threshold. The timedelay may expire before or after the voltage becomes stable, i.e., therate of change of the voltage is substantially zero. The phase “the rateof change of the voltage is substantially zero” means that temporalchange of the voltage is less than 0.1%/ns. The phase “the rate ofchange of the voltage is substantially non-zero” means that temporalchange of the voltage is at least 0.1%/ns.

The controller 710 may be configured to activate the second voltagecomparator during (including the beginning and the expiration) the timedelay. In an embodiment, the controller 710 is configured to activatethe second voltage comparator at the beginning of the time delay. Theterm “activate” means causing the component to enter an operationalstate (e.g., by sending a signal such as a voltage pulse or a logiclevel, by providing power, etc.). The term “deactivate” means causingthe component to enter a non-operational state (e.g., by sending asignal such as a voltage pulse or a logic level, by cut off power,etc.). The operational state may have higher power consumption (e.g., 10times higher, 100 times higher, 1000 times higher) than thenon-operational state. The controller 710 itself may be deactivateduntil the output of the first voltage comparator 701 activates thecontroller 710 when the absolute value of the voltage equals or exceedsthe absolute value of the first threshold.

The controller 710 may be configured to cause the number registered bythe counter 720 to increase by one, if, during the time delay, thesecond voltage comparator 702 determines that the absolute value of thevoltage equals or exceeds the absolute value of the second threshold.

The controller 710 may be configured to cause the voltmeter 706 tomeasure the voltage upon expiration of the time delay. The controller710 may be configured to connect the electrode to an electrical ground,so as to reset the voltage and discharge any charge carriers accumulatedon the electrode. In an embodiment, the electrode is connected to anelectrical ground after the expiration of the time delay. In anembodiment, the electrode is connected to an electrical ground for afinite reset time period. The controller 710 may connect the electrodeto the electrical ground by controlling the switch 705. The switch maybe a transistor such as a field-effect transistor (FET).

In an embodiment, the system 700 has no analog filter network (e.g., aRC network). In an embodiment, the system 700 has no analog circuitry.

The voltmeter 706 may feed the voltage it measures to the controller 710as an analog or digital signal.

The controller 710 may be configured to control the current sourcingmodule 388. For example, the controller 710 may change the magnitude ofcompensation for the dark noise by controlling the current sourcingmodule 788. The controller 710 may adjust the ratio 2040 of the durationof the first compensatory signal 2050 to the duration of a secondcompensatory signal 2060 ratio in the flow of FIG. 13 . The controller710 may execute instructions reside in volatile memory, non-volatilememory, RAM, flash memory, ROM, EPROM, or any other form of anon-transitory computer-readable storage medium and thereby implementthe flows of FIG. 13 , FIG. 14A and FIG. 14B.

FIG. 15 schematically shows a system comprising a radiation detector1600. The radiation detector 1600 may be one exemplary embodiment of oneof the radiation detector described herein. The system may be used formedical imaging such as chest X-ray radiography, abdominal X-rayradiography, etc. The system comprises a pulsed radiation source 2001that emits X-ray. X-ray emitted from the pulsed radiation source 2001penetrates an object 2002 (e.g., a human body part such as chest, limb,abdomen), is attenuated by different degrees by the internal structuresof the object 2002 (e.g., bones, muscle, fat and organs, etc.), and isprojected to the radiation detector 1600. The radiation detector 1600forms an image by detecting the intensity distribution of the X-ray.

FIG. 16 schematically shows a system comprising the radiation detector1600 described herein. The system may be used for medical imaging suchas dental X-ray radiography. The system comprises a pulsed radiationsource 1301 that emits X-ray. X-ray emitted from the pulsed radiationsource 1301 penetrates an object 1302 that is part of a mammal (e.g.,human) mouth. The object 1302 may include a maxilla bone, a palate bone,a tooth, the mandible, or the tongue. The X-ray is attenuated bydifferent degrees by the different structures of the object 1302 and isprojected to the radiation detector 1600. The radiation detector 1600forms an image by detecting the intensity distribution of the X-ray.Teeth absorb X-ray more than dental caries, infections, periodontalligament. The dosage of X-ray radiation received by a dental patient istypically small (around 0.150 mSv for a full mouth series).

FIG. 17 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the radiation detector 1600 described herein.The system may be used for inspecting and identifying goods intransportation systems such as shipping containers, vehicles, ships,luggage, etc. The system comprises a pulsed radiation source 1401.Radiation emitted from the pulsed radiation source 1401 may backscatterfrom an object 1402 (e.g., shipping containers, vehicles, ships, etc.)and be projected to the radiation detector 1600. Different internalstructures of the object 1402 may backscatter the radiation differently.The radiation detector 1600 forms an image by detecting the intensitydistribution of the backscattered radiation and/or energies of thebackscattered radiation.

FIG. 18 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the radiation detector 1600 describedherein. The system may be used for luggage screening at publictransportation stations and airports. The system comprises a pulsedradiation source 1501 that emits X-ray. X-ray emitted from the pulsedradiation source 1501 may penetrate a piece of luggage 1502, bedifferently attenuated by the contents of the luggage, and projected tothe radiation detector 1600. The radiation detector 1600 forms an imageby detecting the intensity distribution of the transmitted X-ray. Thesystem may reveal contents of luggage and identify items forbidden onpublic transportation, such as firearms, narcotics, edged weapons,flammables.

FIG. 19 schematically shows a full-body scanner system comprising theradiation detector 1600 described herein. The full-body scanner systemmay detect objects on a person's body for security screening purposes,without physically removing clothes or making physical contact. Thefull-body scanner system may be able to detect non-metal objects. Thefull-body scanner system comprises a pulsed radiation source 1601. Theradiation emitted from the pulsed radiation source 1601 may backscatterfrom a human 1602 being screened and objects thereon, and be projectedto the radiation detector 1600. The objects and the human body maybackscatter the radiation differently. The radiation detector 1600 formsan image by detecting the intensity distribution of the backscatteredradiation. The radiation detector 1600 and the pulsed radiation source1601 may be configured to scan the human in a linear or rotationaldirection.

FIG. 20 schematically shows an X-ray computed tomography (X-ray CT)system. The X-ray CT system uses computer-processed X-rays to producetomographic images (virtual “slices”) of specific areas of a scannedobject. The tomographic images may be used for diagnostic andtherapeutic purposes in various medical disciplines, or for flawdetection, failure analysis, metrology, assembly analysis and reverseengineering. The X-ray CT system comprises the radiation detector 1600described herein and a pulsed radiation source 1701 that emits X-ray.The radiation detector 1600 and the pulsed radiation source 1701 may beconfigured to rotate synchronously along one or more circular or spiralpaths.

FIG. 21 schematically shows an electron microscope. The electronmicroscope comprises an electron source 1801 (also called an electrongun) that is configured to emit electrons. The electron source 1801 mayhave various emission mechanisms such as thermionic, photocathode, coldemission, or plasmas source. The emitted electrons pass through anelectronic optical system 1803, which may be configured to shape,accelerate, or focus the electrons. The electrons then reach a sample1802 and an image detector may form an image therefrom. The electronmicroscope may comprise the radiation detector 1600 described herein,for performing energy-dispersive X-ray spectroscopy (EDS). EDS is ananalytical technique used for the elemental analysis or chemicalcharacterization of a sample. When the electrons incident on a sample,they cause emission of characteristic X-rays from the sample. Theincident electrons may excite an electron in an inner shell of an atomin the sample, ejecting it from the shell while creating an electronhole where the electron was. An electron from an outer, higher-energyshell then fills the hole, and the difference in energy between thehigher-energy shell and the lower energy shell may be released in theform of an X-ray. The number and energy of the X-rays emitted from thesample can be measured by the radiation detector 1600.

The radiation detector 1600 described here may have other applicationssuch as in an X-ray telescope, X-ray mammography, industrial X-raydefect detection, X-ray microscopy or microradiography, X-ray castinginspection, X-ray non-destructive testing, X-ray weld inspection, X-raydigital subtraction angiography, etc. It may be suitable to use thisradiation detector 1600 in place of a photographic plate, a photographicfilm, a PSP plate, an X-ray image intensifier, a scintillator, or anX-ray detector.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

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 66. A method comprising: measuring signals of an avalanchephotodiode (APD) working in a linear mode when the APD receives noradiation and a compensation for a leakage current of the APD ispresent; when the signals exceed a first level, commencing a time delay;measuring the signals of the APD at an end of the time delay; and whenthe signals at the end of the time delay exceed a second level,increasing the compensation for the leakage current.
 67. The method ofclaim 66, wherein the compensation is increased to a magnitude among agroup of discrete values.
 68. The method of claim 66, furthercomprising: when the signals at the end of the time delay exceed thesecond level, resetting the signals.
 69. A method comprising: measuringsignals of an avalanche photodiode (APD) working in a linear mode whenthe APD receives no radiation and a compensation for a leakage currentof the APD is present; when the signals exceed a first level, commencinga time delay; measuring the signals of the APD at a beginning of thetime delay; measuring the signals of the APD at an end of the timedelay; determining a difference between the signals at the end of thetime delay and the signals at the beginning of the time delay; anddetermining a magnitude of the compensation based on the difference.